Trace Fossils of Shoreshallow Lacustrine Dainan Formation (Paleogene) in Gaoyou Sag, Subei Basin, Jiangsu Province

Key words: 
• trace fossils /
• sedimentary environment /
• Subei Basin /
• Paleogene /
• Dainan Formation

Abstract: The trace fossils in shoreshallow lacustine sediments in Dainan Formation, Paleogene in Gaoyou Sag, Subei Basin consist of at least 6 ichnogenera, 8 ichnospecies, namely, Taenidium barrette, Taenidium satanassi, Beaconites coronus, Cystichnum curvativum, Palaeophycus isp., Planolites montanus, Skolithos verticalis and Skolithos linearis. These trace fossils were mainly of Fodinichnia and Domichnia of invertebrates, and were preserved in full relief and epirelief. Based on the integrated analysis of assembage traits and paleoenvironments, two ichnoassemblages are recognized: 1) TaenidiumBeaconites assemblage, which is mainly of feeding burrows with crescent backfilled structures，and presents itself in shoreshallow lacustrine sediments under periodically arid climate；2）Skolithos linearis assemblage，which is mainly of dwelling burrows，and presents itself in delta plainfront sediments. Based on analysis of bioturbation variation features, three bioturbation assemblages are identified: 1) homogeneous one, which indicates that the water level might rise slowly within little scope and then be stable for a relatively long time; 2) stronger upwards one, which indicates that the water level was generally stable and might rise within little scope, and then be stable, and fall finally; 3) compound one, which indicates that the water level might rise slowly and then fall slowly. In a fan plainfront distributary channel sedimentary sequence, the genres, abundance, diversity of trace fossils and the bioturbation index exhibit regularity vertically. Based on analysis of sedimentology and ichnology, a distributary channel sequence can be divided into 4 sedimentary sets, and each set exhibits different physical and biologic structures.